Such a process is known from patent publication U.S. Pat. No. 7,521,483 B2. This document describes a process for co-producing methanol and ammonia from natural gas and air, comprising the steps of
i. feeding air to an air separation section to make an oxygen (O2) and a nitrogen (N2) stream;
ii. reforming desulphurised natural gas with the O2 stream and steam in a combined reforming section to make a syngas mixture comprising carbon monoxide (CO), carbon dioxide (CO2), steam (H2O) and hydrogen (H2);
iii. Dividing the syngas mixture into a first and a second syngas stream;
iv. Feeding the first syngas stream to a syngas purification section to make a CO2 and a H2 stream;
v. Dividing the H2 stream into a first and a second stream;
vi. Purifying the first H2 stream with the N2 stream to make a pure H2/N2 stream;
vii. Feeding the H2/N2 stream to an ammonia synthesis section to make an ammonia stream;
viii. Feeding the second H2 stream and the second syngas stream to a methanol loop reactor to make a methanol-containing mixture;
ix. Separating crude methanol from the methanol-containing mixture and recycling the remaining gas to the methanol loop reactor;
x. Feeding the crude methanol to a methanol purification section to result in a methanol stream.
It is indicated that this process enables production of up to 5000 mtpd (metric ton per day) of methanol combined with up to 4000 mtpd of ammonia. The process may further comprise reacting the CO2 and NH3 formed into up to 6800 mtpd of urea.
Methanol is one of the most important chemical raw materials; most of the methanol produced is used as a starting material or solvent for synthesis, whereas its use in the fuel and energy sector is expected to increase significantly. Since the 1960's, methanol synthesis from sulphur-free synthesis gas (syngas) with Cu-based catalysts has become the major route, as it can be operated at fairly mild reaction conditions. An overview of methanol processes can be found for example in the chapter “Methanol” in “Kirk-Othmer Encyclopedia of Chemical Technology” (Wiley InterScience; posted on-line 2005/02/18, available via DOI: 10.1002/0471238961.1305200805140712.a01.pub2).
Ammonia is another major chemical raw material, which is used for making urea and other fertilizers, and various chemicals like caprolactam and melamine. It is produced world-wide from nitrogen and hydrogen, typically the hydrogen is obtained via steam reforming of natural gas (or other hydrocarbon feedstock). An overview of ammonia processes can be found for example in the chapter “Ammonia” in “Kirk-Othmer Encyclopedia of Chemical Technology” (Wiley InterScience; posted on-line 2001/10/18, available via DOI: 10.1002/0471238961.0113131503262116.a01.pub2).
For both methanol and ammonia production, it is advantageous—from an economical viewpoint—to develop single line plants with capacity as high as possible. Manufacturing capacity of a single line plant, incorporating only one operating unit or device for each relevant reaction or separation step, is typically limited for technological and economical reasons by a maximum capacity of one or more of its units. Reliability of all units is paramount, as minimizing down-time is a prerequisite for economical operation. For example, a single state-of-the-art air separation unit (abbreviated as ASU) is considered to produce at most about 4000 mtpd (or 5200 kmol/h) of oxygen. Such ASU subsequently limits production capacity of reactors using oxygen as reactant; for example of an auto-thermal reforming (ATR) unit producing syngas from natural gas, steam and oxygen (an ATR is basically a combination of a steam methane reformer (SMR) and a partial oxidation (POX) reactor). Limitations in the maximum size of a SMR unit, on the other hand, lay in the number of reactor tubes. About 1000 tubes is considered to be the maximum for a single unit operation, otherwise it will not be possible to control uniform distribution of gasses and thus heat transfer to all tubes. Further capacity limitation results from a certain maximum amount of energy that can be transferred to the tubes. It is thus estimated that a technically and economically feasible SMR reactor of maximum capacity is currently characterized by a maximum reforming heat load of about 1150 GJ/h. Methanol is typically produced on large scale in a so-called loop reactor, because conversion of syngas into methanol is relatively low. This means that an enormous volume of gas needs to be handled and recycled. For this reason, methanol loop reactors have currently a maximum capacity of 5000-6000 mtpd of methanol.
Integration of methanol and ammonia plants offers further options to reduce costs and boost capacity, by sharing unit operations, internally recycling material streams and re-use of energy (heat). In older processes, a syngas containing carbon oxides (CO and CO2), hydrogen and nitrogen is made, and converted partially to methanol in a methanol loop reactor, methanol is separated from the effluent, and unreacted gas is purified and then fed to an ammonia reactor downstream. An example hereof is given in U.S. Pat. No. 4,367,206, proposing an improvement of such sequential methanol and ammonia co-production using syngas containing carbon oxides, hydrogen and nitrogen as feed, by carrying out the methanol synthesis in two stages, with and without water being present. In DE 3336649 A1 sequential co-production of methanol and ammonia from methane and air is described, wherein the hydrogen/nitrogen ammonia synthesis gas stream is made by reacting excess hydrogen, separated from effluent of the methanol loop reactor, with air.
An integrated process for co-producing methanol and ammonia is also disclosed in U.S. Pat. No. 6,333,014 B1, which process contains the steps of
i. Reforming desulphurised hydrocarbon with steam and air in primary and secondary reformer to make a syngas mixture;
ii. Dividing the syngas mixture into a first and a second syngas stream;
iii. Cooling the first syngas stream to remove a water stream, and feeding remaining syngas to a methanol once-through reactor to make a methanol-containing mixture;
iv. Separating the methanol-containing mixture into crude methanol and methanol-free gas;
v. Feeding the second syngas stream to a high-temperature CO convertor;
vi. Feeding effluent of the high-temperature CO convertor, the methanol-free gas and the water stream to a low-temperature CO convertor;
vii. Feeding effluent of the low-temperature CO convertor to an ammonia synthesis section to make ammonia.
U.S. Pat. No. 5,180,570 also describes an integrated process for co-producing methanol and ammonia from a hydrocarbon feedstock and air, which comprises the steps of
i. separating air into substantially pure O2 and N2 streams;
ii. reforming desulphurised hydrocarbon with steam and O2 in a combined reforming section to make a methanol syngas stream;
iii. Feeding the methanol syngas stream to a methanol loop reactor to make a methanol-containing mixture;
iv. Separating crude methanol from the methanol-containing mixture, and recycling a first part of the remaining gas to the methanol loop reactor;
v. Purifying a second part of the remaining gas and mixing it with N2 to make a ammonia syngas stream;
vi. Feeding the ammonia syngas stream to an ammonia synthesis section to make ammonia.
Considering the increasing demand for fuel and energy, there is a need in industry for ever larger and more efficient methanol and ammonia plants. Presently operated integrated production processes for making methanol and ammonia from hydrocarbon feedstock typically use methanol loop reactors, which have maximum capacity on the order of 5000 mtpd, and need to handle gas volumes about 5 times as much for recycling effluent gas.
There is a continuous need in the industry for a single-line process for making methanol and ammonia in an efficient and economical way, applying unit operations not exceeding current practical capacity limitations (as described above).